This invention relates to granular material spreader units for airplanes for the commonly known "crop duster" operations of spreading fertilizer, insecticides or other chemicals broadly over a field from a flying airplane.
The plane makes one or more passes or flights over the field at a low altitude and discharges the chemical during each pass at a controlled rate of application of granular material to the field. The pilot has a control mechanism for metering the chemical discharge from the plane and for starting and terminating the discharge at the beginning and at the end respectively of each pass. The granular material is carried in a hopper with a spreader unit located beneath the plane's fuselage and the granular material flows through a gap formed by a gate in the spreader unit and at the bottom of the hopper. An operating linkage extends from the gate to the cockpit so that the pilot by operating a lever for the linkage may quickly open or close the gate.
Because of the high cost of the chemical being dispensed and the need for dispensing the chemical uniformly over the field at a recommended rate of coverage, the flow rate control should be precisely controlled or metered. A particular problem in providing a sufficiently precise flow rate is at low rates such as for an insecticide sold under the trade name THIMET which need be applied only at a rate of approximately 5 pounds per acre.
More specifically, the hopper in the plane has a lower discharge opening and the discharge gate across the opening which is controlled by the pilot from within the plane cockpit. When the discharge gate is opened, granular chemical is dispensed from the hopper through a gap formed by the gate into the throat section of a tubular spreader located immediately below the hopper. The spreader has a fantail-shaped discharge at the rear, and the granular material is drawn by the passage of air through the spreader and is uniformly fantailed out to fall then along a strip of an appropriate width across the field. The size of the gap and hence, the flow rate is adjusted by the pilot and this includes an adjustable stop which is positioned to abut the activating lever for opening the discharge gate. That is, this adjustable stop limits the opening movement of the actuating lever to a specified point and the discharge gate is supposed to also form a gap precisely correlated with the stop for the granular material being applied.
Often the granular material is heavy and gritty, and provides in some instances, a resistance to opening movement of the gate, whereby the linkage tends to bend or deflect. This deflection makes for inaccuracies in the flow rate as the gap may be substantially smaller in size than it would be without the deflection. The gap for low flow rates may be only 3/16ths inch. A small deflection from this setting, such as 1/8 inch, changes the flow rate greatly. Also, the linkage is subject to wear and lost motion and other factors which make the gap adjustment most unreliable for some materials. Thus, there is a need for a new improved control of the gap in such machines.